1. Field of the Invention
The present invention relates generally to a technique for bringing a stepping motor to a predetermined home position, and more particularly to such a technique wherein a precise homing control of the stepper motor is effected after a movable element of the motor such as a rotor or a linearly sliding member is mechanically blocked at the home position.
2. Discussion of the Prior Art
U.S. Pat. No. 4,264,220 discloses an apparatus for homing a print wheel of a typewriter, wherein the print wheel is brought to its predetermined home position by mechanically blocking the print wheel at the home position, such that a first stop member provided on a member for rotating the print wheel comes into engagement with a fixed second stop member. According to the disclosed arrangement, the stepper motor is held in an energization phase corresponding to the home position, after the print wheel is mechanically blocked by the second stop member. Thus, the stepper motor is maintained at its home position. Thus, arrangement eliminates a detector for sensing the home position of the print wheel, and contributes to lowering the cost of the typewriter.
The homing method proposed in the above-identified U.S. Patent is widely applicable to a variety of devices or systems wherein a movable member driven by a stepper motor need to be zeroed or placed in its home position. The method is applicable not only to a rotary stepper motor, but also to a linear stepper motor.
Practically, however, the stepper motor cannot be precisely stopped at the predetermined home position simply by mechanically blocking the movable member. The motor can be brought exactly to the home position if it is possible to hold the energization phase corresponding to the home position of the motor, exactly at the moment when the movable element of the motor has been mechanically stopped by engagement of the first and second stop members. In reality, it is difficult to control the timing of energization of the windings in precise synchronization of the mechanical blocking of the print wheel. Usually, the energization phases are changed from one to another for a given time after the movable element has been mechanically stopped. Thus, the stepper motor may fail to be stopped at the predetermined home position.
For example, a rotary stepper motor is operated in a simultaneous two phase energizing mode wherein different combinations of four windings A, B, C and D are energized in a predetermined sequence so as to sequentially establish phases A-B, B-C, C-D and D-A, as indicated in FIG. 9. Suppose the phase C-D of the motor corresponds to the home position of the motor at which the rotor of the motor is mechanically stopped during rotation in the forward direction, the rotor may remain in the home phase or phase C-D position during the next energization of the windings D and A to establish the phase D-A. Alternatively, the rotor may be rotated toward the phase D-A position during the phase D-A energization, even after the rotor is mechanically blocked around the phase C-D position, since there exists a cushioning action of the second stop member upon abutting contact of the first stop member with the second stop member. The cushioning action permits the rotor to be further rotated by a slight angle in the forward direction, and thus allows the rotor to be moved toward the phase D-A position. However, in the next energization of the windings A and B so as to establish the phase A-B, the rotor teeth can not be moved further in the forward direction, and tend to be rotated in the reverse direction as indicated in FIG. 9, while being attracted by the wrong stator teeth of the same phase A-B neighbouring the true phase A-B stator teeth to which the rotor teeth should be attracted in that energization step. Subsequently, the rotor is rotated again in the forward direction as the phases B-C and C-D are established. The above events are repeated, and thus the rotor undergoes oscillating movements to and from the home phase position (phase C-D position) over a given angular range while the energization cycle is repeated, as indicated in solid line in the figure. During such oscillating movements of the rotor, the rotor may jump to the wrong phase C-D position which neighbours the true phase C-D position corresponding to the home position of the rotor, as indicated in broken line in the figure. In this instance, the rotor is erronesouly stopped at the wrong phase C-D position indicated in the broken line. Similar drawbacks are encountered also in a linear stepper motor. In the linear stepper motor, the movable element is a linearly moving member corresponding to the rotor of the rotary stepper motor.